Since the Industrial Revolution, the acidity of the world’s oceans has increased significantly. This change is entirely the result of human activities. About one third of all the carbon dioxide (CO2) emitted by human activities has been absorbed by the oceans. The uptake of CO2 by the oceans produces carbonic acid, altering the chemistry of the oceans and making seawater corrosive to some minerals. Without strong action to reduce CO2 emissions, the oceans will deteriorate to conditions detrimental to shell-forming organisms, coral reefs, and the marine food chain, thus threatening fisheries and marine ecosystems generally. This brief describes the changes in the chemistry of the world’s oceans and explores the potential implications for marine ecosystems and the global food supply.